Activated alumina-dominated phosphors



Dec. 29, 1959 G. E. CROSBY ETAL 2,919,363

ACTIVATED ALUMINA-DOMINATED PHosPHoRs Filed Oct. 5, 1955 0000 0200 lH00 i600 0000 7000 Mn/f man ,4

l a l l i i E s l /Wi/yraes United States Patent' 0 m' ACTIVATED ALUMINA-DOMINATED PHOSPHORS Gilmore E. Crosby, Arthur L. Smith, and La VerneV E.

Whitrner, Lancaster, Pa., assiguors to Radio Corporation of America, a corporation of Delaware Application October 3, 1955, Serial No. 538,039

14 Claims. (Cl. 313-92) This invention relates generally to improved luminescent materials also known as phosphors, and particularly, but not necessarily exclusively, to improved phosphors comprising alumina-dominated host crystals and an activator incorporated therein, to methods for preparing said improved phosphors and to luminescent screens and cathode ray tubes including said improved phosphors.y

An alumina-dominated phosphor is a phosphor wherein the host crystal thereof comprises alumina or alumina in chemical combination with a metal oxide. Activated altuina-dominated phosphors, such as chromium activated alumina, manganese activated zinc aluminate and chromium activated magnesium aluminate have been reported previously.

An object of this invention is to provide improved alumina-dominated phosphors having increased light output under cathode ray excitation.

Another object of this invention is to provide improved methods of preparation of alumina-dominated phosphors.

A further object is to provide improved luminescent screens and improved cathode ray tubes and luminescent means including the improved phosphors of the invention.

In general, the invention includes a new family of activated alumina-dominated phosphors having arsenic incorporated therein. The phosphors of the invention exhibit a greater light output when excited with cathode rays than similar phosphors without arsenic.

A method according to the invention comprises reacting an intimate mixture comprising alumina, an activator and arsenic as an oxide in an oxidizing atmosphere at a temperature below the melting point of the reaction product.

A luminescent screen according to the invention comprises a substrate having a coating thereon including at least one of the phosphors of the invention. Such luminescent screens may comprise a coating of a single composition for producing monochrome light images. For producing polychrome light images, the luminescent screen may comprise a multiplicity of groups of discrete phosphor areas in predetermined order of cyclic succession, said groups including an area comprising a phosphor of the invention.

A cathode ray tube according to the invention, comprises an envelope, a cathode ray gun within one portion of said envelope, and a luminescent screen of the invention in another portion of said envelope, said luminescent screen being adapted to be excited by cathode rays from said gun.

The foregoing objects and other advantages will' be more completely described by reference to the accompanying drawing in which:

Figure l is a curve illustrating the spectral distribution of the emission of a phosphor of the invention when excited with cathode rays; i

Figure 2 vis a partially-sectional, elevational View of a cathode ray tube having a luminescent screen on the 2,919,363 Patented Dec. 29, 1959 2A faceplate thereof including a1 phosphor of; theginvention, an

Figure 3 is a fragmentary explodedl view of` one type of polychrome luminescentscreen having a shadow maskA associated therewith.

Example 7.-Slurry a mixture comprising one mol. of aluminum oxide. 0.091; molf ct Crvzoa. and. 0.5` m01 of As203 in water. Mill the slurry for about 16 hours to obtain intimate mixing of the ingredients, dry at about C. and lire for about one hour at about 1200 C. in air in a quartz crucible. Cool the tir-,ed product, mill in water to provide a fine particle size, andzllhen dry atA about 125 C. The` composition of thev tired product calculated from the composition of theV initial mixtureis A1203 :0.5AS203-1 0;@01C1203.

Referring to Figure 1,v the spectral` distribution ofthe emission in: the visible region of the spectrum of the phosphor of Example 1- when excited with cathode rays is shown by dash-linecurve 21. The spectral distribution of the emission of a similar, phosphor prepared' without arsenic, and similarly excited, is shownv by the solid-line curve 23. The light output in the red portion of the spectrum of thev phosphor of Example 1 prepared with arsenic and excited with cathode r-ays is at least 30% greater than the corresponding composition prepared without, arsenic. I

Example 2.-Slurry a mixture comprising 1.0 mol of BeO, 1.0 mol of A1203, 0.002A mol ofy Cr203I and 0.5 mol of A5203 in water. Mill' the slurry for about 16 hours to obtain intimate mixingV ofthe ingredients, dry at 125 C. and then re for one hour at about 12.50 C. in .air in a quartz crucible. Cool the tired material, mill in water to provide a fine particle size, and then dry at about 125 C. The composition of the tired product calculated from the composition of the initial mixture is BCO 'A1203 I 0.5AS2O3 i 0.002CI'203 However, the light output of the phosphor` of Example 2 is at least. 3.0%. greater than a similarphosphor prepared without arsenic. The color of emission is red. l

Example 3.-Slurry a mixture comprising 1.0 mol of MgO, 1.0 mol of A1203, 0.002 mol of Cr203`and 0.5v mol of As203 in water. Mill the slurry to obtain intimate mixing of the ingredients', dry at about 125 C., and then tire at about l250 C. for about one hour in a quartz Crucible in air. Cool the iired product, mill to obtain a tine particle size and then dry at about 125 C.r The composition of the iired product of Example 3 calculated from the composition of the initial mixture is y 'A1203:0.5AS2O3107002C203 The light output of the phosphor of Example 3 under cathode lray excitation is at least 50% greater than a corresponding composition` preparedA Without arsenic. The color of emission is red.

Arsenic may be used `to intensify the light emission from any alumina-dominatedphosphor. Such phosphor may be an activated alumina .or an activated aluminate of a metal. It is `preferred to intensify phosphors whose host crystals comprise an aluminate of a metal of group Il of the periodic table, `for example, beryllium, magnesium, calcium and barium. The alumina-dominated .phosphors which are improved Vin light output by the inclusion of arsenic may be activated with manganese, chromium, gallium, iron,.silver and .cerium, for example. Arsenic may be added to' the'batch as any oxide of arsenic Aor any salt of arsenic which yieldsvan oxide of arsenic upon heating in air.` However, it isp'referred to add arsenic as. AszOs or asfAs'Z'O. Arsenic may b eadded inactivator proportions `or 'in proportions up to A200 mol percent withrespecttov thehost crystal of thephosphor. ThebatchlisV preferably tired at a v'temperature between `800 C. and

l300 C. However, the batch may be tired at any temperature below the melting point of the phosphor and suciently high to recrystallize the host crystal material and to incorporate the activator therein. The raw batch is preferably iired in an oxidizing atmosphere although a neutral atmosphere may be used in many cases. Analysis of the tired product indicates that substantially all of the arsenic of the raw batch is incorporated in the fired product.

The ingredients for preparing the phosphors of the invention are discussed as oxides and may be introduced into the raw batch as raw oxides. However, the ingredients may also be introduced as compounds of the various metals which yield oxides of these metals upon heating in air. Thus, for example, one may use carbonates, nitrates, acetates, hydroxides, hydrates or oxalates of these metals. The ingredients are preferably non-volatile at elevated temperatures and of the highest purity.

Example 4.-A preferred green-emitting phosphor that is particularly useful in color television may be prepared by the following method. Slurry a mixture of 0.45 mol of ZnO, 0.50 mol of A1203 as aluminum hydroxide, 0.06 mol of As205 and 0.01 m01 of manganese as manganous sulfate in distilled water. Mill the slurry to obtain intimate mixing of ingredients, dry at 160 C., and then iire at about 1l00 C. in air for about 1 hour. Cool the f ired product, mill in carbon tetrachloride to provide a tine particle size and then dry at about 125 C. The composition of the tired product calculated from the composition of the raw batch is The light output of the phosphor of Example 4 under cathode ray excitation is at least twice as great as a corresponding composition prepared without arsenic.

The preferred green-emitting manganese activated zinc aluminate compositions may be expressed by the following molecular formula: aZnObA12O3tcAszdMn wherein a is between 0.41 and 0.49, b is between 0.59 and 0.51, c is between and 0.12, a' is between 0.001 and 0.05 and a-l-b equals l.

Referring to Figure 2, cathode ray tubes according to the invention may include a luminescent screen comprising a substrate having a coating therein comprising a phosphor of the invention. The cathode ray tube may comprise a tube base 49 including a cathode ray gun 51, a glass envelope comprising a neck portion 53, a conical portion 55 and a faceplate 57. A thin layer 59 of a composition including a phosphor of the invention is disposed on the inner surface of the faceplate 57. A thin layer 59 on the faceplate 57 is referred to as the luminescent screen. The thin Ylayer 59 may comprise a uniform composition throughout or may comprise a plurality of discrete areas comprising a phosphor of the invention. Where the thin layer 59 comprises a uniform composition, the cathode ray tube is referred to as having a monochrome screen. Where the luminescent screen comprises a plurality of areas having phosphors of different emission color characteristics, the luminescent screen is referred to as a polychrome screen.

Referring to Figure 3, there is shown one form of tricolor'kine'scope screen, which is a type of polychrome screen. The screen comprises a transparent glass base plate 61 having arranged on one side thereof a coating 63 comprising a multiplicity of closely spaced circular phosphor areas or dot trios. Each trio includes a dot R of a red-emitting phosphor, such as copper-activated zinc-cadmium selenide. Each trio further includes a dot B of a blue-emitting phosphor which may comprise a silver-activated zinc sulfide. Each trio further includes a. dot` G of` a green-emitting phosphor such as described in Example 4. The centers of these three dots of each trio lie at the corners of an equilateral triangle. The centers ofthe trios themselves lie at the corners of equilateral triangles of larger size.

Spaced in front of the plate 6l is a shadow mask 673 provided with a multiplicity of holes 69, each hole 69' being associated with and spaced from a particular trioI on the luminescent screen. The mask 67 and the coatingl 63 may be used in a kinescope having means for simultaneously projecting three electron beams, the axis ofl which are shown as dotted lines '71, 73 and 75. These beams are located 120 apart about the tube axis and are converged to a point at the plane of the mask 67. The convergence angle of the beams, the positioning of the mask 67 with respect to the base plate 61 and the arrangement of the holes 69 in the shadow mask with respect to the phosphor dot trios, all cooperate to scan each electron beam across the screen to excite only phosphor dots in each trio emitting light of one particular color.

The above-type of screen is only one of a number ol diierent arrangements of polychrome luminescent screens. The general type is a screen which comprises discrete phosphor areas emitting light in each of lthree different colors, regardless of the particular shape of each of the areas and their specific geometrical arrangement on the screen, at least one of said areas comprising a phosphor of the invention. If intended for use in the usual type of television kinescope, the base plate should be of glass and the phosphor areas are normally disposed on one surface of the plate. 1f the screen is to be viewed by front surface light rather than transmitted back surface light, the base plate or at least its surface, may be an opaque material such as a ceramic, a glass or a metal. By base plate is meant in general, al substrate material on which the phosphor areas are disposed, whether the plate be planar or non-planar, whether it be a single member or a plurality of members and whether the phosphor areas be disposed on one side or more than one side thereof. Also included in the invention are tri-color luminescent means comprising three monochrome screens each having ditferent light emission characteristics. The light emission from each of the screens is projected upon a single surface producing a composite color image. All of the luminescent screens comprise a general category of luminescent means having in common a combination of one or more phosphors of different emission characteristics, the emission of which is combined in such a way as to produce polychromatic images. Tllc phosphors of the type described in Example 4 are useful in tri-color luminescent means for color television. They are especially useful when all three component phosphors of the luminescent means have a short persistence.

There have been described improved phosphors of thc alumina family and improved methods of preparation `thereof. By incorporating arsenic in alumina-dominated phosphors according to thel invention, the phosphors exhibit an increased light output under cathode ray excitation. v Such phosphors are useful in improved pol"- chrome luminescent means and improved cathode ray tubes including the improved phosphors of the invention.

What is claimed is:

1. A phosphor consisting essentially of a host crystal selected from the group consisting of alumina, beryllium aluminate, magnesium aluminate, calcium aluminate, strontium aluminate, barium aluminate, zinc aluminate, and cadmium aluminate, an activator selected from the group consisting of manganese, chromium, gallium, iron, silver, and ceum, and between 0.1 yand 200 mol percent with respect to said host crystal material of arsenic incorporated therein.

2. A phosphor consisting essentially of manganese-activated zinc aluminate having arsenic incorporated therein in proportions between 0.1 and 200 mol percent with respect to said zinc aluminate.

3. VA phosphor consisting essentially of chromium-activated alumina having arsenic incorporated therein in proportions between 0.1 and 200 mol percent with respect to said alumina.

4. A phosphor consisting essentially of chromium-activated magnesium aluminate having arsenic incorporated therein in proportions between 0.1 and 200 mol percen with respect to said magnesium aluminate.

5. A phosphor consisting essentially of chromiumactivated beryllium aluminate having arsenic incorporated therein in proportions between 0.1 and 200 mol percent with respect to said beryllium aluminate.

6. A method for preparing a luminescent material comprising tiring a mixture compresing alumina, an activator selected from the group consisting of manganese, chromium, gallium, iron, silver, and cerium, and between 0.1 and 200 mol percent with respect to said alumina of arsenic, as a compound thereof, at a temperature between 800 C. and l300 C. inv an oxidizing atmosphere.

7. A method for preparing a luminescent aluminate comprising tiring a mixture consisting essentially of alumina, an oxide of a metal selected from the group consisting of beryllium, magnesium, calcium, strontium, barium, zinc, and cadmium, an activator selected from the group consisting of manganese, chromium, gallium iron, silver, and cerium and between 0.1 and 200 mol percent with respect to said aluminate of arsenic, as an oxide, at a temperature between 800 C. and 1300 C. in an oxidizing atmosphere.

8. A luminescent screen comprising a substrate having a coating thereon including a phosphor consisting essentially of a host crystal selected from the group consisting of alumina, beryllium aluminate, magnesium aluminate, calcium aluminate, strontium aluminate, barium aluminate, zinc aluminate, and cadmium aluminate, an activator selected from the group consisting of manganese, chromium, gallium, iron, silver, and cerium and between 0.1 and 200 mol percent with respect to said host crystal of arsenic incorporated therein.

9.- A luminescent screen comprising a substrate having a coating thereon including a phosphor consisting essentially of manganese-activated zinc aluminate having arsensic incorporated therein in proportions between 0.1 and 200 mol percent with respect to said host crystal.

l0. A luminescent screen for color television comprising a base plate having disposed on a surface thereof a multiplicity of groups of discrete phosphor areas arranged in a predetermined order of cyclic succession, said groups including a green-emitting component consisting essentially of a manganese-activated zinc aluminate having arsenic incorporated therein in proportions between 0.1 and 200 mol percent with respect to said zinc aluminate.

11. Luminescent means for a color television reproducer comprising discrete areas of different phosphors, each phosphor area capable of emitting light in one of three dierent colors when excited, one of said phosphors consisting essentially of a manganese-activated zinc aluminate having arsenic incorporated therein in proportonsbetween 0.1 and 200 mol percent with respect to said zinc aluminate.

12. A cathode ray tube comprising an envelope, a cathode ray gun within one portion of said envelope and a luminescent screen comprising a substrate having a coating thereon including a phosphor consisting essentially of a host crystal selected from the group consistingof alumina, beryllium aluminate, magnesium aluminate, calcium aluminate, strontium aluminate, barium aluminate, zinc aluminate, and cadmium aluminate, an activator selected from the group consisting of manganese, chromium, gallium, iron, silver and cerium, and between 0.1 and 200 mol percent with respect to said host crystal of arsenic incorporated therein within another portion of said envelope, said luminescent screen being adapted to be excited by cathode rays from said cathode ray gun.

13. A cathode ray tube comprising an envelope, a cathode ray gun within one portion of said envelope and, within another portion of said envelope, a luminescent screen comprising a substrate having a coating thereon including a phosphor consisting essentially of manganeseactivated zinc aluminate having arsenic incorporated therein in proportions between 0.1 and 200 mol percent with respect to said zinc aluminate, said luminescent screen being adapted to be excited by cathode rays from said cathode ray gun.

14. A cathode ray tube comprising an envelope, a cathode ray gun within one portion of said envelope, and within another portion of said envelope, a luminescent screen comprising a base plate having disposed on a surface thereof a multiplicity of groups of discrete phosphor areas arranged in a predetermined order of cyclic succession, said groups including a green-emitting phosphor consisting essentially of a manganese-activated zinc aluminate having arsenic incorporated therein in proportions between 0.1 and 200 mol percent with respect to said zinc ,aluminate, said luminescent screen being adapted to be excited by cathode rays from said cathode ray gun.

References Cited in the tile of this patent UNITED STATES PATENTS 2,116,167 Espig May 3, 1938 2,206,280 Froelich July 2, 1940 2,312,266 Roberts Feb. 23, 1942 2,392,814 vFroelich Jan. 15, 1946 2,398,629 Fonda Apr. 16, 1946 2,523,026 Jones Sept. 19, 1950 2,580,073 Burton Dec. 25, 1951 2,590,411 Isenberg Mar. 25, 1952 OTHER REFERENCES Jones: Journal of Electrochemical Society, vol. 95, No. 6, VJune 1948, pages 295-298.

Some Aspects of Luminescence of Solids, by Kroger Elsevier Pub. Co., New York (1948), pp. 49-52, 262-270. 

1. A PHOSPHOR CONSISTING ESSENTIALY OF A HOST CRYSTAL SELECTED FROM THE GROUP CONSISTING OF ALUMINA, BERYLLIUM ALUMINATE, MAGNESIUM ALUMUNATE, CALCIUM ALUMINATE, STRONTIUM ALUMINATE, BARIUM ALUMINATE, ZINC ALUMINATE. CADMIUM ALUMINATE, AN ACTIVATOR SELECTED FROM THE P CONSISTING OF MANGANESE, CHROMOUM, GALLIUM IRON. R, AND CERIM AND BETWEEN 0.1 AND 200 MOL PERCENT 